This invention relates generally to air intake silencers for use with internal combustion engines, and, more particularly, to air intake silencers for use with outboard motors.
Internal combustion engines typically include an air intake system for receiving combustion air that is mixed with fuel and combusted in the engine cylinders. Noise from the engine, however, also typically travels through the air intake system to the atmosphere. In certain engines, such as, for example, a two-stroke outboard motor, noise travelling from the engine through the air intake is a significant noise source when the engine is operated at high speeds.
To mitigate engine noise that travels through the air intake, two stroke outboard motors are often equipped with air intake silencers including expansion chambers or resonance chambers to attenuate engine noise traveling through the air intake. Due to size constraints in outboard motor constructions, however, known air intake silencers are of limited effectiveness. Typically, known air intake silencers produce attenuation of less than 4 dB, and are generally ineffective at frequencies below 500 Hz.
In an exemplary embodiment of the invention, an air intake silencer includes at least one air inlet pipe comprising a first end, a second end, and a passage therethrough, and at least one tuning tube in fluid communication with the air inlet passage. The tuning tube includes a first end, a second end, and a passage therethrough that extends for a length selected to cancel noise of at least a first selected frequency passing through the air inlet pipe.
More specifically, the tuning tube and the air inlet pipe have passages of substantially equal diameters, but the passages extend for different path lengths through the air inlet pipe and the tuning tube. The path length difference causes half wavelength cancellation of a selected frequency of sound exiting from the air inlet pipe from an engine through the air intake silencer. In a further embodiment, the air intake silencer includes a plurality of tuning tubes located in a wrap-around relationship with one another to tune different frequencies and produce half wavelength cancellation of more than one frequency. The air inlet pipe and tuning tube may be integrally formed, and in different embodiments may be formed into an air intake manifold that silences more than engine air inlet. In one embodiment the air intake silencer is integral to a motor cover.
The above-described air intake silencer achieves broad band noise reduction of about 10 dB to about 20 dB in a frequency range of about 300 Hz to about 800 Hz.